The present invention relates to the radio telephone art. Specifically, an image reject mixer is provided which draws a minimum of current to preserve the battery life for the radio telephone.
Image rejection mixers solve a common problem of interfering signals experienced by cellular radio telephone receivers. Using a hetrodyne conversion system, the conversion of the input radio frequency signal to an intermediate frequency signal may simultaneously produce undesirable mixing products called spurs which interferes with the conversion of the signal of interest. The most common type of spur encountered in the frequency conversion is an image, i.e., the product of converting an adjacent signal at a lower frequency to a mixing product which interferes with the immediate frequency signal produced by the signal of interest.
A common technique used to reject spurs or images is to use a filter that prevents the radio frequency signal adjacent the radio frequency of interest from entering the frequency conversion circuitry. Filters, however, are large, costly, and may consume an undesirable amount of current from the battery. Another approach to the image problem is to use special phasing techniques which cancel out the image. These image reject mixers, while effective in rejecting the image, also suffer from high current consumption which reduces the battery life. It is therefore desirable to provide an image rejection mixer which can operate at low current, thereby saving battery power, while preserving the lineary and noise performance of the frequency conversion circuit.
In accordance with the invention, first and second doubly balanced mixers are provided which convert a common radio frequency signal to an intermediate frequency signal. The first doubly balanced mixer produces a pair of differential currents which represent the sum and difference of the local oscillator signal and a radio frequency signal. A second doubly balanced mixer frequency converts the same radio frequency signal producing a pair of differential currents representing the sum and difference of the radio frequency signal, using a ninety degree phase shifted version of the local oscillator signal. The first and second pairs of differential signals are combined in a quadrature combining circuit to produce a differential intermediate frequency signal substantially free of signal images.